1. Field of the Invention
The invention generally relates to the manufacturing of glass ceramic articles, such as glass ceramic plates for cooktops or fireplace windows. More particularly, the invention relates to the manufacturing of at least partially transparent glass ceramic articles.
2. Description of Related Art
Glass ceramic plates find application as cooktops, inter alia. The glass ceramics used for this purpose typically exhibit low to zero thermal expansion of usually less than 1.5×10−6/K in a temperature range between room temperature and the operating temperature of up to 700° C. In one embodiment, these glass ceramics are transparently dyed throughout their volume in order to hide the components of the hearth arranged below the cooktop. In a second embodiment, the glass ceramics are transparent and the technical components below the glass ceramic can be hidden from view by opaque, i.e. light-blocking coatings on the lower and/or upper surfaces. This transparent embodiment of the glass ceramic is also used for fireplace window applications.
And, for practical or aesthetic reasons it has been desired to enable luminous display elements to shine through the glass ceramic plate. In this case, high transmission and low color distortion are desirable. In the transparent embodiment, a good and unaltered view on a colored lower surface coating, for example, or on the flames in the fireplace should be ensured.
WO 2010/040443 A2 discloses a transparent, dyed cooktop with improved color display capability, which comprises a glass ceramic that includes high quartz mixed crystals as a predominant crystal phase, and the glass ceramic is free of chemical refining agents arsenic oxide and/or antimony oxide, except for inevitable traces. The glass ceramic exhibits transmittance values of greater than 0.1% in the region of visible light in the entire range of wavelengths greater than 450 nm, with a light transmittance in the visible region ranging from 0.8 to 2.5% and in the infrared region at 1600 nm ranging from 45 to 85%.
In order to achieve the coloration, vanadium oxide, V2O5, and iron oxide are added as color-imparting oxides.
Vanadium oxide has a very strong coloring effect. Therefore it is added in appropriately low amounts. This implies that adjustment of an intended transmittance is critical, since small deviations of the vanadium content in the glass mixture will cause strong alterations in the color appearance of the glass ceramic. This problem is exacerbated further when comparatively high transmittance values are to be achieved in the visible spectral range, since this even increases the relative variations of the vanadium oxide content at a given uncertainty in the dosing of the mixture.
Another problem is the adjustment of a specific content of coloring oxide in the preparation of the starting glass in a continuous melting process. If, in this case, it is desired to produce a glass for a glass ceramic that has a higher transmittance, for example, the content of coloring oxide would have to be reduced. However, this is only possible in a complex remelting process in which a large amount of unuseable glass is produced.
It would therefore be desirable to facilitate the adjustment of a specific hue or a specific absorptivity of the glass ceramic in the visible spectral range.